Plastic parts can be made using an injection molding process. In the injection molding process, a molding material is injected under pressure into a mold cavity, where it solidifies in the shape of the mold cavity to form the molded part. Injection molding has a high capital investment because a steel or aluminum mold cavity is costly and time consuming to fabricate. Since there is a high capital investment, injection molding is generally not suited for small quantity fabrication, and since the mold cavity is time consuming to fabricate, injection molding generally is not suited for the early prototype stages of development of a part where significant design changes may be rapidly occurring. Accordingly, it would be advantageous to be able to inexpensively and quickly develop a mold cavity, particularly for the early prototype stages of development.
Rapid prototyping methods (e.g., Stereolithography, Laminated Object Manufacturing, Selective Laser Sintering, and Fused Deposition Modeling) have been employed to provide objects. Stereolithography is an example of a layer-additive process to enable the rapid prototyping generation of a three-dimensional object from a CAD (Computer Aided Design) database. A series of closely spaced horizontal planes are passed through the database to represent a series of closely spaced 2-dimensional cross-sections, each a different Z coordinate value. Referring to FIG. 1, a movable light source such as a laser 10, typically a computer controlled ultraviolet helium-cadmium or argon ion laser, traces the cross-sections of the computer-generated model onto a surface 12 of a vat of a laser curable liquid polymer 14, thereby hardening/solidifying the material. After the first layer is completed, a platform 16 holding the formed object 18 is lowered, leaving a new layer of the liquid polymer over the cured (i.e., hardened) material equal in thickness to the cross-section trace made by the laser. The process is repeated until the object is complete, whereby the platform is elevated and the solidified molded plastic 3-dimensional object emerges from the vat. The object 18 is placed in a post cure apparatus where it is flooded with ultraviolet radiation to effect a thorough post cure. An advantage of this process is the speed at which a computer-generated design may be turned into a three-dimensional object. The reference "Stereolithography and other RP&M Technologies; from Rapid Prototyping to Rapid Tools", by Paul F. Jacobs, ASME Press, New York, 1996, provides additional information on stereolithography.
Accordingly, it would be advantageous to employ a layer-additive process (such as stereolithography) to produce an object which is a mold cavity for producing injection molded parts. Such a mold cavity could then be employed to mold a limited quantity of parts suitable for the prototype stages of development. Questions about a molded part or molding material can be determined early in the design process by parts molded from a mold cavity fabricated using stereolithography.
Since some injection molded parts include particular features, a mold cavity produced using stereolithography must provide the particular features. For example, the mold cavity must be able to provide for part removal--that is, the mold cavity must incorporate features for removing the molded part from the mold cavity. Further features may include passageways of small size and movable components.